Nervous System💪🏻 Flashcards
What are the steps in a response?
- Stimulus
- Receptor
- CNS
- Effector (muscle or gland)
- Response
What are neurones?
- Basic functional unit of the nervous system
* Highly specialised cells that are able to generate and transmit nerve impulses
What is the cell body? (Neurones)
Contains nucleus, nucleolus and other organelles (including ribosomes - neurotransmitters and mitochondria - Na+/K+ pump
What is a dendrite? (Neurones)
- Thin cytoplasmic extensions which carry impulses towards the cell body
- Relatively short in motor neurone and able to communicate with other neurones
What is an axon? (Neurones)
- Carries impulses away from the cell body
- Form connections with a muscle of gland at motor end plates
- Numerous mitochondria in branched ends - involved in synthesis of transmitter substances
- Myelinated axon has a fatty sheath of myelin
What are Schwann cells? (Neurones)
- Wrap themselves around the axon, along its length
* Results in several layers of fatty myelin surrounding axon
What are the nodes of Ranvier? (Neurones)
- Gaps between adjacent Schwann cells
* Here, axon is exposed - no myelin
What is the effect of the myelin sheath?
Increases the rate of transmission of impulses along the axon
What is the difference between sensory and motor neurones?
- Sensory - one long dendrite bringing information
* Motor - one long axon taking information away
What is the spinal cord?
A hollow tube running from the base of the brain to the end of the spine
What is grey matter? (Spinal cord)
- Central
* Contains cell bodies of relay and motor neurones
What is white matter? (Spinal cord)
- Outer
* Contains myelinated axons which run up and down the spinal cord, to and from the brain
What is the spinal canal? (Spinal cord)
- Centre of the grey matter
* Nutritive cerebrospinal fluid circulates through this
What is the dorsal root ganglion? (Spinal cord)
- Sensory neurones enter spinal cord via dorsal root
* Concentration of their cell bodies forms a swelling
What is the ventral root? (Spinal cord)
Motor neurones leave via ventral root
What is a reflex?
- An automatic, rapid response to an adverse stimulus
* The neurones that are involved in making a reflex occur make up a reflex arc
How is the resting potential generated?
•In a resting axon: -high conc. of Na+ outside -high conc. of K+ inside •Membrane is polarised •Net effect - inside is negative compared to outside giving the resting potential •K+ leakage channels - K+ leaks out •Na+/K+ pump - 3Na+ out, 2K+ in •Negative particles within axon
What is meant by threshold intensity?
- Action potential can only be generated if the stimulus reaches a certain threshold intensity
- Below this threshold, no action potential can be created
- Once the threshold level is reached, the size of an impulse is independent of the intensity of the stimulus
What is the membrane potential difference at resting potential? (mV)
-70mV
What is the link between a strong stimulus and action potentials?
A stronger stimulus produces a greater frequency of action potentials
What is the refractory period?
- Time delay following the passage of one action potential
- Lasts a few milliseconds
- Depolarisation can’t happen - Na+ channels are closed which prevents inward movement of Na+ so another impulse can’t be conducted
- After this, repolarisation occurs - K+ channels open
What is the importance of the refractory period?
•Impulses can only flow in one direction along an axon
- so the region of axon behind the impulse can’t be depolarised
•Limits the frequency at which successive impulses can pass along an axon
What are nerve impulses?
- Info carried in the form of electrochemical signals
- Travel as action potentials and pass along axon in a wave of depolarisation
- Results In changes in potential difference across the axon membrane
What are voltage gated proteins?
- Respond to changes in voltage
* Opened by depolarisation
What is the effect of depolarisation? (Active neurone)
- Na+ voltage gated proteins open
- Na+ ions diffuse in down a concentration gradient
- Voltage becomes +40mV
What is the effect of repolarisation? (Active neurone)
- Na+ voltage gated proteins close
- K+ channels open
- K+ ions flood out of axon
- Brings it back to -70mV
What is the effect of hyperpolarisation?
- K+ channels remain open
- Inside of axon becomes too negative (-80mV)
- Once these channels close, resting potential can be restored
What does the transmission speed of an impulse depend upon?
- Axon diameter
* Myelin sheath
How does an increased axon diameter increase the speed of impulse transmission?
- Greater S.A. of axon membrane over which exchange of ions can occur
- Also reduces resistance - current can glow further, increasing the length of the local circuit
What are giant axons?
- Found in a number of invertebrates
- Thought to be involved with rapid escape responses
- RapId transmission of impulses is needed between receptors and muscles to withdraw the animal from danger
How does myelination increase the speed of impulse transmission?
- By electrically insulating the axon
- Impermeable to Na+ and K+ so depolarisation and action potentials can’t occur at these points
- CAN occur at nodes of Ranvier - saltatory conduction which is faster
What is saltatory conduction?
When the action potential jumps from one node to the next (in a myelinated axon)
What are the advantages of saltatory conduction?
- Increases speed of impulse transmission by up to 100 times
- Conserves energy - Na+/K+ pump only operates at nodes so fewer ions have to be transported across membrane to restore resting potential
How may increased temperature affect transmission speeds?
- Increases rate of respiration
* Restoring resting potential requires energy from ATP - Na+/K+ pump
Stage 1 of synaptic transmission of acetylcholine (Calcium ions)
- Action potential arrives
- Depolarisation of presynaptic membrane
- Ca+ gated channels open
- Ca+ ions flood into knob/bulb down a conc gradient
Stage 2 of synaptic transmission of acetylcholine (Vesicles)
- Influx of Ca+ ions causes vesicles containing acetylcholine to move towards the presynaptic membrane
- Vesicles fuse with membrane
- Release neurotransmitter into cleft
Stage 3 of synaptic transmission of acetylcholine (Receptors)
- Acetylcholine diffuses across cleft by exocytosis
* Binds to specific receptor proteins in postsynaptic membrane (receptors are attached to gated Na+ channels
Stage 4 of synaptic transmission of acetylcholine (Sodium ions)
- Na+ gated channels in postsynaptic membrane open up due to binding of acetylcholine cashing protein to change shape
- Na+ flood into membrane
- Membrane becomes depolarised which initiates an action potential
- Impulse travels to next neurone
Stage 5 of synaptic transmission of acetylcholine (Enzyme)
- To prevent continuous production of action potentials, acetylcholinesterase (enzyme) is present in synaptic cleft
- Enzyme hydrolyses acetylcholine into acetate and choline
- Choline and acetate diffuse back into knob
- Energy from ATP is used to resynthesise acetylcholine from acetic acid and choline
Why are mitochondria needed in the presynaptic bulb/knob?
To produce ATP for the resynthesis of acetylcholine
What is temporal summation? (Functions of the synapse)
- Each action potential that arrives at presynaptic membrane will cause a number of vesicles to release their transmitter - accumulation of neurotransmitter
- A number of action potentials are required before there is enough transmitter (threshold level) to initiate an action potential in the postsynaptic cell - temporal summation
What is spatial summation? (Function of the synapse)
- A number of presynaptic neurones may form synapses with one postsynaptic neurone
- Action potentials arriving in each presynaptic neurone will release a transmitter, which builds up to threshold level and triggers a postsynaptic impulse - spatial summation
Why can impulse only pass a synapse in one direction? (Functions of the synapse)
Synaptic vesicles are only present in the presynaptic knob/bulb
What effect do organophosphates have on synaptic transmission?
- Inhibit acetylcholinesterase
- Acetylcholine is not hydrolysed and remains in cleft
- Repeated firing of post-synaptic neurone
What is the effect of psychoactive drugs on synaptic transmission?
- Affect different neurotransmitters or their receptors - bind and block at the receptors, preventing neurotransmitters from binding
- Affects the firing of neurones
- This alters the brain function
Example of phylum in which the organisms have nerve net systems
phylum Cnidaria
What is a nerve net?
- Simplest type of nervous system
* A diffuse network of cells that group into ganglia but do not form a brain
What are the two types of cell in a nerve net?
- Ganglion cells - provide connections in several directions
- Sensory cells - detect stimuli
Hydra vs Human
•Nervous system type: Hydra - nerve net Human - CNS •No. of cell types in N.S. Hydra - 2 Human - many •Regeneration Hydra - rapid Human - very slow if at all •Myelin sheath Hydra - absent Human - present •Conduction speed Hydra - slow 5ms-1 Human - fast 120ms-1 •Ability to regenerate neurones Hydra - present Human - absent
How is an action potential generated?
•When neurone is stimulated, voltage gated Na+ channels open
- Na+ ions flood in
•Membrane is depolarised
•Potential difference across membrane is briefly reversed, becoming positive (40mV) on the inside
•Change In polarity is known as an action potential
What is the all or nothing law?
Above the threshold, the size or strength of the action potential is always the same, irrespective of the size or strength of the stimulus
Describe the propagation of a nerve impulse
- Action potential at one end opens active gated Na+ channels, causing depolarisation here
- Depolarisation affects next part, which also depolarises and original region begins to repolarise. Na+ channels at original position are shut and inactivated
- Repeats down the axon
- Original region is now fully repolarised and process happens again (with a distance between the other depolarised region)
Why do myelinated neurones use less ATP than non-myelinated neurones?
The sodium potassium pumps are only active at the nodes of Ranvier
What are the two main types of neurotransmitter?
- Acetylcholine
* Noradrenaline
What are excitatory drugs?
•Similar shape to neurotransmitter and bring about same effect
•May inhibit enzyme that breaks down the neurotransmitter
-remains attached to postsynaptic membrane
What are inhibitory drugs?
- Bind to and block receptors on the postsynaptic membrane
- Prevents neurotransmitters from binding
- Can cause paralysis or death due to inability of muscles to contract
